In 3rd Generation Partnership Project (3GPP), work is ongoing on specifications of the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network evolution (UTRAN). This work is frequently referred to as evolved UTRAN (E-UTRAN), as part of the Long Term Evolution (LTE) effort.
In LTE, the scheduler is placed in the eNodeB (eNB) and the Medium Access Control (MAC) layer. The scheduler assigns radio resources, also called Resource Blocks (RB), for the downlink (assignments) as well as for the uplink (grants) using the Physical Downlink Control Channel (PDCCH). Also, information concerning which transport format to use is comprised within the assignment and grant, respectively.
The radio downlink is the transmission path from a base station, e.g. an eNodeB, to a User Equipment (UE). The uplink is the inverse of a downlink, i.e. the transmission path from the user equipment to the base station.
For uplink scheduling, the eNodeB needs information about the current state of the buffers in the user equipment i.e. if and how much data the user equipment has in its priority queues. This information is sent from the user equipment to the eNodeB either as a 1-bit scheduling request (SR) or by a buffer status report (BSR). Buffer status reports are transmitted on a data channel such as Physical Uplink Shared Channel (PUSCH), mostly together with user data. Before access to the data channel is granted, scheduling requests are transmitted on a control channel such as e.g. Physical Uplink Control Channel (PUCCH) or Random Access Channel (RACH). If the user equipment has a valid PUCCH resource for scheduling request configured in any Transmission Time Interval (TTI) it sends a one bit scheduling request when the timing is right.
Scheduling requests may either be transmitted on the RACH channel (RA-SR) or on dedicated resources on the PUCCH (Dedicated SR, D-SR) if such resources are available. The PUCCH resources for dedicated SR are assigned and revoked by the eNodeB through Radio Resource Control (RRC). In addition, the resources are autonomously revoked when the user equipment looses uplink synchronization.
A dedicated scheduling request is typically used when the user equipment uplink is time synchronized. The purpose is to enable user equipment to rapidly request resources for uplink data transmission. In 3GPP, a dedicated solution for the scheduling request has been agreed on. For the dedicated approach, each active user is assigned a dedicated channel for performing the scheduling request. The benefit with this method is that no user equipment identity has to be transmitted explicitly, since the user equipment is identified by the channel used. Furthermore, no intra-cell collisions will occur in contrast to the contention based approach.
Precise and up-to-date scheduling information allows more accurate scheduling decisions, and can help to optimize the use of radio resources and to improve capacity. However, the accuracy of the information provided by the user equipment is limited by the granularity of the buffer status reports, by the frequency of the scheduling requests and buffer status report transmissions and by the delay between the reception of the scheduling requests or buffer status report and the scheduling decision.
For delay sensitive services with periodical packet arrival, such as Voice over the Internet Protocol (VoIP), the likelihood that the buffer status information is outdated when it is used is high. It is likely that additional data has arrived since the buffer status report was transmitted. It is also likely that the buffer will be emptied frequently and therefore the only available information will be a one bit scheduling request. With only a 1-bit indication from the user equipment, it is impossible for the eNodeB to know what kind of data that has arrived in the user equipment's buffer. This means further that the eNodeB scheduler might not be able to prioritize important data, such as handover signalling messages, even though this data is associated with a Quality of Service (QoS) Class Identifier (QCI) of high priority. This will result into an unendurable delay for the important data.
With incorrect uplink information, the scheduler is furthermore likely to provide either a too large grant, which then result in the user equipment transmitting padding and may reduce system capacity, or a too small grant, which may lead to Radio Link Control (RLC) segmentation and an increase in transmission delay.
Buffer status reporting is used by the user equipment to report to the eNodeB the amount of data stored in its buffers for transmission. The eNodeB uses these reports to allocate resources to the user equipment, and to prioritize resource allocation among different user equipments.
The user equipment triggers a regular buffer status report and scheduling request when uplink data becomes available for transmission and if this data belongs to a Logical Channel Group (LCG), or radio bearer group, with higher priority than those for which data already existed in the buffer or if the user equipment buffers were empty just before this new data became available for transmission.
The dedicated scheduling request is repeatedly transmitted on consecutive scheduling request opportunities on PUCCH until the user equipment receives an uplink grant on PDCCH. The transmission is stopped at least when PUCCH resources are released and/or uplink synchronization is lost even if the user equipment has not received any uplink grant on PDCCH. After stopping transmission on the dedicated scheduling request, the user equipment transmits on the random access scheduling request, i.e. accesses the system via RACH.
The random access scheduling request is used when the user equipment has lost uplink synchronization or if it has no dedicated scheduling request resources.
With the current approach in LTE, the eNodeB can with a received scheduling request only determine that the user equipment has new data to be scheduled, without any further knowledge on the amount and type of data available. This means for example that the eNodeB in many cases do not know to what QoS class identifier the available data is related and with what priority it thereby should be treated in scheduling decisions. This can in turn result in that important traffic with high priority is not recognized as such and therefore not treated with the required priority.
In a scenario where a service such as VoIP is used, where the user equipment buffer is typically emptied frequently and the only information for the eNodeB scheduler will be the reception of a 1-bit scheduling request, the scheduler cannot differentiate between the arrival of a VoIP frame and a high priority signalling message related to e.g. handover. Handover signalling messages are important to treat with high priority as the handover signalling is typically triggered when the user equipment experiences bad channel conditions at the cell border and the signalling is therefore urgent to complete. Long scheduling delays may lead to that the user equipment moves further away from the serving cell and into worse channel conditions, possibly leading to that the messages cannot be delivered at all. The handover may therefore fail due to the eNodeB's inability to differentiate VoIP traffic from handover signalling.
Only having a 1-bit scheduling request means further that the eNodeB has no knowledge of the amount of data available in the user equipments buffer. The scheduler can therefore in this case only make a “best guess” when assigning transmission resources, which is likely to lead to suboptimal use of the available resources.
Previously known scheme in LTE for scheduling request transmissions has the following shortcomings:
The only information the reception of a scheduling request gives is that new data is available in the user equipment buffer. It is therefore in many cases impossible for the eNodeB to determine which Logical Channel Group (LCG), and thus also QCI, the data belongs to and with which priority it should be handled. Upon receiving a scheduling request, the eNodeB has no knowledge on the amount of data available in the buffer of the user equipment and it is therefore likely that the assigned transmission resources are not the most optimal. A method for obtaining more information on the user equipment buffer status with the reception of a scheduling request may be useful for improving the scheduling.